The magnetotelluric (MT) method is an established technique which uses measurements of naturally occurring electromagnetic fields to determine the electrical resistivity, or conductivity, of subsurface rocks. An MT survey employs time series measurements of orthogonal components of the electric and magnetic fields, which define a surface impedance. This impedance, observed over a broad band of frequencies and over the surface, determines the electrical conductivity distribution beneath that surface, with horizontal layers of the earth being mathematically analogous to segments of a transmission line. Principal factors affecting the resistivity of subsurface materials include temperature, pressure, saturation with fluids, structure, texture, composition and electrochemical parameters. Resistivity information may be used to map major stratigraphic units, determine relative porosity or support a geological interpretation. A significant application of MT surveying is oil exploration. An MT survey may be performed in addition to seismic, gravity and magnetic data surveys. A combination of data from two or more different survey methods leads to a more complete understanding of subsurface structure than may be possible through the use of any single technique alone, particularly where the structure is such that measurement using a given technique may be contraindicated. For example, certain structures such as sediments buried under salt, basalt or carbonate have poor seismic performance and productivity. These structures generate strong reflections and reverberations, making imaging of the buried sediments difficult using acoustic methods alone. On the other hand, because the MT method does not involve the measurement of responses to artificially-created seismic events, it can be utilized in lieu of or in combination with seismic methods to minimize the error induced by reflections.
While the MT method has been used on land as an aid to petroleum exploration for many years, its application to marine continental shelf exploration is more recent. Remote reference data acquisition, robust data processing and multidimensional modeling and inversion have been required to produce meaningful responses to signals available from MT surveys.
Significant progress has been made in recent years in the ability to collect seafloor MT data. In the past, one primary reason for the limited use of MT for seafloor mapping was that high frequencies are rapidly attenuated by seawater, leading to a dramatic loss of electric and magnetic field power on the seafloor at periods shorter than 1000 seconds. However, to be useful for mapping continental shelf structures at depths relevant to petroleum exploration, MT measurements should be made at periods between 0.1 and 1000 seconds. The present inventor has addressed these difficulties by developing a system and method disclosed in U.S. Pat. No. 5,770,945, issued on Jun. 23, 1998, and entitled “Seafloor Magnetotelluric System and Method for Oil Exploration”, which is commonly assigned with the present application and incorporated by reference herein. The system and method disclosed in that patent utilize horizontal electric and magnetic field measurements to map seafloor geological structures of interest in oil prospecting. An important improvement provided by the disclosed system and method is derived from the use of higher frequencies, on the order of 1 Hz, making it more appropriate for petroleum exploration compared with the systems in the prior art.
Measurement of horizontal fields alone provides only part of the information that is desirable for detailed mapping of geologic structures. While vertical electric fields do not exist at the surface on land, they are present in the ocean. Therefore, in order to fully exploit the MT technology for seafloor exploration, it would be advantageous to provide for measurement of vertical magnetotelluric impedances, which are preferentially sensitive to lateral variations in geological structure. Previous attempts to measure vertical e-fields have used noisy, tethered instruments, e.g., electrodes attached at intervals along a length of flexible cable which is dragged behind a vessel. For example, see U.S. Pat. No. 4,047,098 of Srnka or U.S. Pat. No. 2,839,721 of DeWitte. These prior art systems also used low frequency amplifiers such that they were subject to the above-described frequency limitations for application to seafloor oil exploration.
An additional use of the instrument described in U.S. Pat. No. 5,770,945 is for controlled source electromagnetic surveying, in which an electromagnetic (EM) transmitter is placed or towed in seawater, preferably at or near the seafloor. The amplitude and phase of the transmitted signals are then used to determine the electrical resistivity of subsurface rocks and fluids. Controlled EM source methods are well known in the art and have become almost routine for mapping of electrical conductivity of the seafloor in very shallow to deep ocean water, achieving seafloor penetration depths as great as 30 km in 5 km of water. However, these known methods have failed to recognize and/or take advantage of the vertical electric field and its ability to measure lateral variations in seafloor subsurface structure.
Accordingly, the need remains for a system and method which can exploit a vertical field's ability to measure lateral variations in the geological structure of the seafloor.